Three days before the rockets fell outside Damascus, a team of Syrian specialists gathered in the northern suburb of Adra for a task that U.S. officials say had become routine in the third year of the country’s civil conflict: filling warheads with deadly chemicals to kill Syrian rebels.

The preparations, as described by U.S. intelligence analysts, continued from Aug. 18 until just after midnight on Aug. 21, when the projectiles were loaded into rocket launchers behind the government’s defensive lines. Then, at 2:30 a.m., a half-dozen densely populated neighborhoods were jolted awake by a series of explosions, followed by an oozing blanket of suffocating gas.

Unknown to Syrian officials, U.S. spy agencies recorded each step in the alleged chemical attack, from the extensive preparations to the launching of rockets to the after-action assessments by Syrian officials. Those records and intercepts would become the core of the Obama administration’s evidentiary case linking the Syrian government to what one official called an “indiscriminate, inconceivable horror” — the use of outlawed toxins to kill nearly 1,500 civilians, including at least 426 children.

Pulling back the curtain on some of the United States’ most sensitive collection efforts, the Obama administration released on Friday its long-awaited intelligence assessment of the Aug. 21 event, explaining in rare detail the basis for its claim that Syria was behind the release of deadly gas, the grisly effects of which have been documented in more than 100 amateur videos.

The four-page assessment and accompanying map revealed for the first time how communications intercepts and satellite imagery picked up key decisions and actions on the ground.

Official U.S. government report on the alleged use of chemical weapons by the Syrian government against rebels on Aug. 21.

In choosing to release the document, White House officials anticipated the likely comparisons to the famously inaccurate intelligence reports from a decade ago that claimed that Iraqi President Saddam Hussein was actively pursuing nuclear, chemical and biological weapons. Secretary of State John F. Kerry, in his remarks on the release of the intelligence assessment, said White House officials were “more than mindful of the Iraq experience.”

“We will not repeat that moment,” Kerry said. “Accordingly, we have taken unprecedented steps to declassify and make facts available to people who can judge for themselves.”

The document proposes a possible motive for the attack — a desperate effort to push back rebels from several areas in the capital’s densely packed eastern suburbs — and also suggests that the high civilian death toll surprised and panicked senior Syrian officials, who called off the attack and then tried to cover it up.

“We intercepted communications involving a senior official intimately familiar with the offensive,” it says, “who confirmed that chemical weapons were used by the regime on Aug. 21 and was concerned with the U.N. inspectors obtaining evidence.”

The Obama administration has “high confidence” that Syria used chemical weapons to kill civilians based on a dossier of evidence, Secretary of State John F. Kerry said Friday. (The Washington Post)

While unusually detailed, the assessment does not include photographs, recordings or other hard evidence to support its claims. Nor does it offer proof to back up the administration’s assertion that top-ranking Syrian officials — possibly including President Bashar al-Assad — were complicit in the attack.

“There is additional intelligence that remains classified because of sources and methods concerns,” the report says. “That is being provided to Congress and international partners.”

Among the surprises in the report was the U.S. estimate for the dead and wounded. The new figure, 1,429, was about four times higher than a British casualty estimate released Thursday.

“This assessment will certainly evolve as we obtain more information,” the report said.

The material, prepared by senior intelligence officials, was said to reflect the judgments of the CIA, National Security Agency and other intelligence agencies involved in gathering information on the Syrian conflict. Using understated phrasing typical of bureaucratic reports, it asserts with “high confidence” that the Assad government launched a chemical weapons attack, using what it said was “nerve agent,” a class of chemical munitions that includes sarin.

Echoing the findings of a British intelligence assessment a day earlier, the report links the Assad government to “multiple” chemical weapons attacks in the past year, including a small-scale attack in the same part of the Damascus area that was struck Aug. 21.

It suggested that a relatively controlled use of chemicals had in recent months become part of the normal military strategy whenever government ­forces were unable to push back rebel offensives or break through defensive fortifications.

“The Syrian regime has used chemical weapons over the last year primarily to gain the upper hand or break a stalemate in areas where it had struggled to seize and hold strategically valuable territory,” it says. “We assess that the regime’s frustration with its inability to secure large portions of Damascus may have contributed to its decision to use chemical weapons on Aug. 21.”

Indeed, in the week before the attack, government troops had been repeatedly frustrated in their efforts to dislodge rebels from dozens of villages east of Damascus, despite the use of aircraft, helicopters and heavy artillery barrages. The fighting appeared to have stalled when, on Aug. 18, U.S. intelligence agencies observed a team of chemical weapons specialists being activated in Adra, a northern suburb “near an area that the regime uses to mix chemical weapons, including sarin,” the document states. The ordnance team was attached to Syria’s Scientific Studies and Research Center, a military-run institute linked to Syria’s chemical and nuclear weapons programs.

On Aug. 21, regime forces were observed making further preparations for a chemical attack, “including through the utilization of gas masks,” according to the intelligence summary. Soon afterward, satellites and other surveillance aircraft picked up the flashes of rockets and artillery shells being fired from government positions toward rebel-held and contested villages east and south of the city. The first reports of a gas attack appeared on social-media sites soon after the projectiles landed.

As the scale of the civilian casualties was becoming clear, U.S. monitors made another key intercept, this time a message to a Syrian chemical weapons team that was “directed to cease operations,” the U.S. assessment says. Later that day, other Syrian troops began an intensified barrage on the same neighborhood using conventional artillery in an apparent effort to cover up the evidence.

“In the 24-hour period after the attack, we detected indications of artillery and rocket fire at a rate four times higher than the ten proceeding days,” the four-page document states.